Volume scatter shader causes black spots
These are caused by self-intersecting bits of the cloud particles showing the backsides of faces to the outside world:

Rays going through the back of a face don't count as going into the volume, and so appear mostly transparent.
To fix this you have a couple options:

Lighten up on the displacement to avoid such intersections:


Or add a remesh modifier (and optionally a decimate modifier to keep the vertex count down)